In the present description, the invention will be described more particularly in its non-limiting application to third generation radio communication networks of the universal mobile telecommunication system (UMTS) type. In this system, the invention finds application within the framework of the high speed uplink packet access (HSUPA) feature being specified by the 3rd Generation Partnership Project (3GPP)—also named “FDD enhanced uplink” in 3GPP terminology, or “E-DCH” according to the transport channel's name. This feature is described particularly in the technical specification TS 25.309, V6.6.0, “FDD Enhanced Uplink; Overall description; Stage 2 (Release 6)”, published in March 2006 by the 3GPP.
FIG. 1 shows the architecture of such a UMTS network. The switches 101 of the communication network belonging to a core network (CN), are linked on the one hand to one or more fixed network 103 and on the other hand, by means of a so-called lu interface, to command equipment 105 or radio network controllers (RNCs). Each RNC 105 is linked to one or more base stations (BSs) 107 (or Node-Bs according to 3GPP specifications) by means of a so-called lub interface. The BSs 107, distributed over the territory covered by the network, are capable of communicating by radio with the mobile terminals 109 called user equipments (UEs). Certain RNCs 105 may furthermore communicate with one another by means of a so-called lur interface. The RNCs 105 and the BSs 107 form an access network called UMTS terrestrial radio access network (UTRAN).
The UTRAN comprises elements of layers 1 and 2 of the ISO model with a view to providing the links required on the radio interface (called Uu), and a stage 201 (FIG. 2) for controlling the radio resources (radio resource control, RRC) belonging to layer 3, as is described in the 3GPP TS 25.301 technical specification “Radio Interface Protocol Architecture”, version 6.4.0 published in September 2005 by the 3GPP. In view of the higher layers, the UTRAN acts simply as a relay between the UE and the CN.
FIG. 2 shows the RRC stages 201a, 201b and the stages of the lower layers which belong to the UTRAN and to a UE. On each side, layer 2 is subdivided into a radio link control (RLC) 203a, 203b and a medium access control (MAC) stage 205a, 205b. Layer 1 comprises a coding and multiplexing stage 207a, 207b. A radio stage 209a, 209b caters for the transmission of the radio signals from trains of symbols provided by the stage 207a, 207b, and the reception of the signals in the other direction.
There are various ways of adapting the architecture of protocols according to FIG. 2 to the hardware architecture of the UTRAN according to FIG. 1 and in general various organisations can be adopted depending on the types of channels (see section 11.2 of the 3G TS 25.401 technical specification “UTRAN Overall Description”, version 7.0.0 published in March 2006 by the 3GPP). The RRC, RLC and MAC stages are typically located in the RNC 105. When several RNCs 105 are involved, the MAC sublayer can be apportioned among these RNCs 105, with appropriate protocols for the exchanges on the lur interface, for example asynchronous transfer mode (ATM) and ATM adaptation layer No. 2 (AAL2). These same protocols may also be employed on the lub interface for the exchanges between the MAC sublayer and layer 1.
UMTS proposes a high speed uplink packet access (HSUPA) feature of which an overall description can be found in the 3GPP 25.309 technical specification “FDD enhanced uplink; Overall description; Stage 2”, version 6.6.0 published in March 2006 by the 3GPP. HSUPA allows high rate uplink transmission, i.e. from a UE to the access network. This service is based on the so-called enhanced dedicated channel (E-DCH) and it has the following characteristics: a new type of transport channel which supports hybrid processes for requesting data retransmission of the hybrid automatic repeat request (HARQ) type, adaptive modulation and coding and BS scheduling of the uplink data transmissions.
At the MAC level, a new MAC entity (MAC-es/MAC-e) has been added in the UE below MAC-d as seen in FIG. 3. MAC-es/MAC-e handles scheduling and MAC-e multiplexing, HARQ retransmissions and E-DCH traffic format combination (TFC) selection. The MAC-e, has been introduced in the UTRAN architecture, and more specifically at the BS level to handle HARQ retransmissions, scheduling and MAC-e demultiplexing. In the serving RNC (SRNC) MAC-es is added to provide in-sequence delivery (reordering) and to handle combining of data from different BSs in case of soft handover. This architecture is illustrated in FIG. 3, and described in the technical specification TS 25.309 “FDD eenhanced uplink; Overall description; Stage 2”, version 6.6.0, published in March 2006 by the 3GPP.
In the downlink, a resource indication (scheduling grant) is required to indicate to the UE 109 the maximum size of a transport bloc, mapped onto a modulation and coding scheme (MCS) that the UE 109 can use. When issuing scheduling grants, the BS 107 may use quality of service (QoS) related information provided by the SNRC and from the UE 109 scheduling requests. There are two types of grants. Absolute grants provide an absolute limitation of the maximum amount of uplink resources the UE may use. The absolute grant is transmitted in the downlink cell on a physical channel called enhanced absolute grant channel (E-AGCH). FIG. 4 illustrates this situation. Relative grants on the other hand increase or decrease the resource limitation compared to the previously used value.
As far as the power control of a radio channel in a communication system is concerned, a simple solution would be to use constant power for E-AGCH transmissions. However, if the transmission power is set to a too low value, the information transmitted on the channel may not be correctly received by the receiver. For instance, in the context of E-AGCH, it is important that the UE 109 receives information sent by the BS 107 on E-AGCH correctly so that the UE 109 knows when to transmit on E-DCH and which MCS to apply. On the other hand, the constant power, as is used for instance on E-AGCH, wastes network resources and creates interference, if it is set to a too high value.